NODULE PERMEABILITY TO O-2 AND NITROGENASE-LINKED RESPIRATION IN BEANGENOTYPES VARYING IN THE TOLERANCE OF N-2 FIXATION TO P-DEFICIENCY

Since nodule permeability that contributes to regulate the nodule oxyg en concentration at a low level, has been shown to vary with P nutriti on in soybean, the effect of P deficiency on nodule permeability was i nvestigated in common bean (Phaseolus vulgaris L.) genotypes, G19839 a nd G3010, that were previously selected for their contrasting response of nodule nitrogenase activity to P deficiency. Shoot dry weight for control P treatment was higher in G3010 than in G19839, while that for low P treatment was higher in G19839 than in G3010, although root and nodule biomass were almost similar in both genotypes whatever P treat ment. For both genotypes, low-P plants formed fewer nodules with a hig her proportion of small nodules than in control plants. Total O-2 upta ke of nodulated root (JOrn) and nodule O-2 uptake linked to nitrogenas e (JOn) were respectively 2 and 1.5 times higher in control than in lo w-P plants in both genotypes, and O-2 uptake per gram nodule dry weigh t was increased by 140 and 84% in G3010 and G19839, respectively, by P deficiency. The nodule O-2 permeability, computed as the ratio of the slope of the regression between JOrn and external pO(2) to the total nodule surface, did not differ between genotypes for control plants. B y contrast, nodule O-2 permeability was increased by P deficiency in t he sensitive genotype G3010 while it remained unchanged in the toleran t genotype G19839. In the sensitive genotype, P deficiency also decrea sed the nodule respiratory quotient. It is suggested that in the sensi tive cultivars, P deficiency increases oxygen limitation in the nodule s by increasing the rate of O-2 consumption that may cause an increase in nodule permeability to alleviate this nodule O-2 limitation.